Metal-air and particularly zinc-air battery systems are known in the art and due to their high energy densities, relative safety of operation and the possibility of ready mechanical recharging, such systems have been suggested as a power source for electrically propelled automotive vehicles. To date, for various reasons, such systems have yet to meet with significant commercial success.
One of the principle drawbacks of battery systems as a power source for automotive vehicle propulsion, and particularly zinc-air battery systems, resides in the difficulty in achieving the combination of both a high continuous current drain, such as is needed for extended uphill driving, with short term high peak power output such as is needed for quick acceleration, while at the same time maintaining a high energy density and facilitating rapid rechargeability.
On the one hand, in order to achieve high continuous current drain a large reservoir of active anode material is needed. Due to space and other considerations this is generally best achieved by incorporation of a highly porous active anode element having large-surface active anodic material.
By contrast, in order to achieve high peak power output, i.e. the ability to provide a very high level of power for short bursts of time, studies have found that a tight interparticulate structure of the active anodic material is advantageous. This comes at the expense of the porosity of known powdered anodes and can drastically reduce the current capacity of the battery.
To date, in known-in-the art battery systems much emphasis has been placed on achieving high capacity. Zinc anodes in various battery systems are generally formed in one of two broad families of processes: According to one family, particularly applicable to primary alkaline batteries, the anodes are constructed from finely powdered zinc typically produced by a thermal atomization process. The resultant zinc powder typically has a particulate size distribution of between 0.0075 to 0.8 mm and a surface area of between 0.2-0.4 m.sup.2 /gr; it is generally combined with mercury, sodium carboxymethyl cellulose and KOH solution to form a gelled mass readily extruded into an anode form. Alternatively the powdered zinc may be sintered, or wetted with mercury and pressed into a plate. Porosity of the anode may be controlled by use of removeable pore forming materials such as NH.sub.4 Cl. The density of the zinc anode material produced by such methods is typically within the range of 2.5-3.5 gr/cc.
According to a second family of processes, exemplified by an anode proposed by Ross, U.S. Pat. No. 4,842,963, claimed to be particularly suitable for electrically rechargeable zinc-air batteries, the electrode is prepared by directly electro-plating metallic zinc from a solution of zinc ions onto a current collector. The electroplating process may be done external to the battery cell, or in secondary battery applications; within the cell itself. The current collector may be in the form of a metallic plate, metallic mesh, metal foam or conductive carbon foam. Alternatively, a zinc electrode is prepared by pasting a mixture of zinc oxide and plastic binder, typically teflon, onto a current collector; the zinc oxide is then electroformed to zinc directly on the current collector within the cell.
At typical current densities appropriate to use in electric vehicles it has been found that zinc-air batteries in which the anodes are constructed according to the above methods fail to provide a combination of rapid rechargeability, high current capacity and high peak power output. Hence it would be desirable to develop an anode capable of providing a battery with all of these attributes, i.e. high current density, rapid rechargeability, high current capacity and high peak power output.
In an effort to further increase the advantages of using zinc-air battery systems for electro-automotive propulsion, it has been further proposed to employ a mechanically rechargeable battery system comprising a rigid anode designed to be rapidly removed and replaced on a periodic basis at service stations specifically equipped for the purpose. The spent anodic material, which after use has been oxidized, may then be recycled external to the battery for later reuse in other batteries. To facilitate recycling of the active anodic material, it is necessary to separate the spent anodic material from the other anode components.
It would therefore also be desirable to provide an anode easily removeable from the battery cell, in which the active anodic material is readily separable from the supporting anode structure so as to facilitate recycling of the anodic material external to the cell.